In the future, in view of the emissions burden that is caused by traffic, hydrogen will be used increasingly as a fuel for motor vehicles, aircraft and ships. The storage of hydrogen on board these vehicles is suitably carried out in liquid form, since based on the low density of gaseous hydrogen, the storage capacity would otherwise be only very limited. The hydrogen is therefore cooled to about 25 K and introduced into the storage tank that is in the vehicle at a pressure of 3 to 4 bar.
If a combustion engine is used as a drive assembly, the filling pressure of 3 to 4 bar is just enough for proper operation of the engine. When fuel cells are used to power vehicles, however, at this time a hydrogen supply under a pressure of 10 bar is necessary. Storage of hydrogen at a pressure of 10 bar and a corresponding equilibrium temperature of about 31 K is disadvantageous, however, since the storage capacity clearly drops because the density of the liquid hydrogen decreases with rising temperature.
In practice, therefore, the hydrogen that is stored at a pressure of 3 to 4 bar is first compressed to 10 bar, before it is fed to the fuel cell. The pressure increase can be achieved by, for example, introducing additional gaseous hydrogen into the storage tank or by evaporating a portion of the liquid hydrogen. Such a device for pressure build-up is known from, for example, DE 42 12 626 A1. In this connection, a gaslift is attached to the bottom of the storage tank, in which liquid hydrogen that enters into the gaslift with use of the pump heating is partially evaporated. The gas bubbles that are produced in this case entrain liquid hydrogen via a rising main upward into the gas chamber of the storage tank, where the latter is ultimately evaporated with use of an evaporator-heating system.